1. Field of the Invention
The invention relates generally to the field of electromagnetic methods in geological formation exploration and evaluation. More particularly, the invention relates to a high sensitive magnetic sensing assembly for measuring magnetic field produced by a current flowing in the earth formations energized by an electromagnetic energy source. The invention also relates to the application of transient electromagnetic method, magnetometric resistivity, deep induction well logging and induction cross-hole imaging.
2. Background Art
In electromagnetic geophysical exploration and formation evaluation, electromagnetic signals are received and processed to provide information on conductivity distribution of the earth formations. The receiving sensors used for these purposes are typically either induction coils or magnetometers. Induction coils produce signal proportional to time rate of change of magnetic field. Magnetometers measure magnetic field flux density starting from static magnetic field. Typically the magnetic fields to be measured are produced by time-varying electric currents with a broad frequency spectrum.
An example of applications demanding a broadband magnetic receiver is transient electromagnetic measurements described in U.S. Pat. No. 5,955,884 issued to Payton et al. A remarkable advantage of transient electromagnetic instrument is an ability to separate in time the response of different spatial areas. In order to facilitate measurement of the formation response from nearby to deep areas both the electromagnetic transmitter and magnetic receiver should have substantially broad operating frequency band. In addition, the magnetic receiver should have high sensitivity to low frequency magnetic field corresponding to signals coming from deep areas. Yet another application that requires high sensitivity and a broad frequency band is cross-well induction imaging based on transient measurements.
Neither of the existing magnetic sensors can cover all the range of possible rates of the magnetic field changes, so a different sensors for different spectral ranges are to be used. State of the art of induction coils for geophysical applications in different frequency ranges is represented by products of Shlumberger, EMI Technology Center. The coils comprise a soft magnetic core, winding and a built-in low noise preamplifier. A relatively high frequency coil model BF-10 covers frequency range of 0.1 Hz-10 kHz.
An induction coil for low frequency range 0.0001-1000 Hz is the BF-4 model having minimum noise spectral density 0.1 pT/√{square root over (Hz)} at 1 Hz. The noise at lower frequency goes up fast and reaches 200 pT/√{square root over (Hz)} at 0.001 Hz. A low frequency coil normally requires a big ferromagnetic core and dozens of thousands of turns of winding limiting the upper operating frequency. The limit is caused by the self-resonance of the winding due its stray capacitance. It also requires very low noise amplifier at low frequency where low noise is hardly to achieve due to 1/f-type noise of an operational amplifier. Another example of a very low frequency coil can be found in U.S. Pat. No. 4,901,023 issued to Vail, III, in application to induction logging of cased wells. The coil uses a bulky magnetic core and 30,000 turns coil. Its high frequency is limited to only about 20 Hz.
Better performance for extremely low frequency may be expected from magnetometers. One of the best types of magnetometers that can be practically used for the electromagnetic measurement in geophysical application is a fluxgate magnetometer. The principle of operation of fluxgate magnetometers can be found for example in N. F. Ness, Magnetometers for space research, Space Science Review 11 (1970) 459-554. An example of a very low noise fluxgate magnetometer is TFM-100 model of Billingsley Magnetics. The noise spectral density at 1 Hz is 7 pT/√{square root over (Hz)}. At around 1 Hz frequency the fluxgate magnetometers normally can not offer as low noise as the low frequency induction coils. On the other hand it has substantially slower noise increase at very low frequency. The frequency range where fluxgate magnetometers may compete with low frequency induction coils is 0.0001-0.1 Hz. Another advantage of the fluxgate magnetometer is much smaller dimensions and weight.
In a variety of applications there is still a need for a high sensitive magnetic receiver capable of sensing a broadband magnetic field signals. The magnetic sensing assembly should be preferably small and light. To provide a solution for this problem is an objective of the present invention.